Are all SDTV subchannels anamorphic?

[Walter Dnes]

I notice that SDTV subchannels rescale slightly when launching. I can receive some on my HDHomerun networked tuner, and play back on computer using mplayer. The diagnostics from mplayer include...

Starting playback...
Movie-Aspect is 1.33:1 - prescaling to correct movie aspect.
VO: [xv] 704x480 => 704x528 Planar YV12

I leave it at 704x528 on my notebook (1366x768 display), and double up each dimension to 1408x1056 on my desktop (1920x1200 dsiplay). Both machines are linux. So far, I've confirmed this on NBC Universal Sports (2.2 Buffalo), RTV (2.3 Buffalo) and TCN (29.2 Buffalo). WNED and WNYO aren't coming in for me tonight. Anyone else notice this rescaling? How about SDTV subchannels from other cities?

[GeorgeMx]

The rescaling is just converting the non-square pixels to square pixels for computer display. If the vertical pixel count is 480, the horizontal should be 640 (computer VGA). The 704x480 conversion to 704x528 produces square pixels while retaining the horizontal resolution of 704 pixels (704/528=1.333). The standard SD format is 720x480 but Motorola encoders and possibly others create 704x480.
The pixel aspect ratio for SD is all over the map. The 720 horizontal pixels per line was fixed when the studio serial digital interface (SDI) was created in the 1980s. NTSC 525 line and PAL/SECAM 625 both use 720 horizontal pixels but the vertical lines count can be 480, 486 or 576. The total data rate for 525 and 625 balances with the frame rates of 29.97 and 25 frames per second. Any of the horizontal/vertical ratios can represent a 4x3 or 16x9 picture aspect ratio. Fortunately, all the HD formats use square pixels.